In the latest post to his site Marc Morera about the use of "lazy services" with Symfony2. In his examples, he uses a command line application to illustrate his point, but it could apply elsewhere as well.

Since Symfony version 2.4 you can define your controllers and commands as services. This is so useful as long as you need to treat your classes as much decoupled as possible. [...] When we define as lazy a service, this is not instanced when is injected, but only when is accessed. [...] The point here is to define our service intended to work with the model as lazy.

He shows how to implement this kind of "lazy" handling in a command, registering the commands into the services but not creating the instances of them until they're used. He includes some example code showing how this is set up and offers a few tips on the implementation and common issues to think about.